(1) Field of the Invention
This invention relates to a novel coating rubber composition usable for tire cords wherein swelling in extrusion is reduced and dimensional stability and modulus of elasticity are improved by compounding short fibers of syndiotactic (hereinafter abbreviated as syn)-1,2-polybutadiene into a rubber.
(2) Description of the Prior Art
Recently, savings of resources and energy become a significant subject in industries. Particularly, saving of fuel consumption in vehiecles by weight-saving of tire and reduction of tire rolling resistance is significantly interested in tire industry. For this end, there has hitherto been studied a method wherein the weight-saving of tire is carried out by increasing strengths of materials to be used in tread and bead portions of the tire, but such a weight-saving has already reached the limit.
Now, attemps have been made to reduce a weight of a coating rubber composition for cords in a carcass and a breaker. However, it is very difficult to make a thickness of a rubber coating thin and uniform. For instance, when the coating rubber composition is extruded through an orifice of an extruder, an extrudate having a shape larger than that of the orifice is obtained due to an elasticity inherent to an unvulcanized rubber. This phenomenon is usually called as a swelling. If the swelling is large, the thickness of the rubber coating covering the cord becomes thicker locally and hence it is difficult to uniformly coat the cord with the coating rubber composition. Therefore, it is usually performed that the thickness of the rubber coating is slightly thickened in advance. Thus, it is an essential feature that the swelling of the rubber coating is small in order to make the thickness of the coating uniform and thin.
Moreover, even if it is possible to uniformly and thinly coat the cords with the coating rubber composition, when a carcass or breaker is made from a plurality of such rubberized cord layers, a distance between the cords in the adjacent upper and lower layers becomes small and hence an interlaminar shearing stress applied to the rubber between the cords becomes large and consequently it is apt to cause a fracture of rubber. Therefore, it is necessary to use a rubber capable of enduring the interlaminar shearing stress.
Up to now, however, there is not yet realized a coating rubber composition having a small degree of swelling, a high strength and an excellent durability.